A polyphase alternating current (AC) dynamoelectric machine can be used as a motor or a generator. In aeronautical applications, it is desirable to use a single machine for a starter motor and a generator to reduce size and weight. An aircraft generator can be used as a motor to start the propulsion engine for the aircraft when it is powered by an inverter.
To reduce cost and improve reliability, it is very desirable for the engine starter to eliminate mechanical shaft sensor. In general, there are two categories in sensorless motor control, the back EMF based method and the signal injection method. The back EMF based method is easy to implement, and usually works quite well at high angular rotor velocity, but it is inadequate for low velocity or standstill. The signal injection method is more difficult to implement, but it is preferred for operation at low angular rotor velocity or standstill. Most systems that utilise the signal injection method are also subject to a 180 degree rotor position anomaly because these systems are not able to recognise if they are locking onto the positive or negative pole of the rotor.